The present invention relates to a head for injecting liquid under pressure to excavate the ground, and in particular it relates to an injection head for implementing the technique known as xe2x80x9cjet groutingxe2x80x9d.
The jet grouting technique consists in breaking up the soil by means of a jet of liquid having very high kinetic energy that is implemented in a borehole, the liquid jet serving to erode the ground in which excavation is to be performed. To form the jet, a nozzle is used which is fixed to the end of a drill string, the rods of the drill string serving simultaneously to convey liquid under high pressure to the nozzle(s) and to move the nozzle progressively in translation in the soil. More precisely, the nozzle(s) is/are mounted on a member usually referred to as a xe2x80x9cmonitorxe2x80x9d or as an xe2x80x9cinjection headxe2x80x9d, which member is fixed to the bottom end of the drill string, said monitor itself optionally being fitted at its bottom end with a mechanical boring tool. As is known, the liquid generally used is a cement-based slurry which serves, after boring, to make a cement element in the ground that is molded in place in the soil.
The liquid which is conveyed by the drill string is delivered from the surface by means of a pump at a pressure of one to several tens of megapascals (MPa). The inside diameter of the drill string conveying the liquid must be large enough to minimize head losses in the string. This diameter can typically be of the order of 20 millimeters (mm) to 50 mm. In contrast, the outlet diameter from the nozzle must be small enough to impart sufficient speed to the jet of liquid that leaves the nozzle for it to erode the soil remotely. Typically, the outlet diameter of the nozzle lies in the range 2 mm to 5 mm as a general rule, and the outlet speed of the liquid from the nozzle lies in the range one to several hundreds of meters per second (m/s).
To obtain a jet of high quality, it is desirable for the inside shape of the nozzle to be optimized so as to conserve as high a speed as possible for the liquid jet as it moves away from the nozzle towards the soil so as to enable it to erode the soil as much as possible while using a minimum amount of kinetic energy. Optimized nozzle shapes satisfying this requirement are in widespread use.
However, even with such nozzles, it is found that the jet quickly loses effectiveness in eroding the soil, such that considerable levels of kinetic energy are necessary so that when the drill string is moved in translation and optionally in rotation, the soil is eroded at a considerable distance away from the nozzle, e.g. at a distance of several decimeters (dm). The active radius of the jet of liquid under pressure for forming a column, a sector of a column, or a plane element generally remains poor, lying in the range a few decimeters to 1 meter (m) or 2 m depending on the method implemented, on the nature of the soil, and on the energy deployed.
To increase the action of the jet, proposals have been made, in particular in U.S. Pat. No. 5,228,809, to implement the injection head or monitor in such a manner as to improve the quality of the jet.
Accompanying FIG. 1 shows the injection head described in that patent. The injection head 10 comprises a body 12 having a side wall 14 which defines an inside cavity. A nozzle 16 for injecting liquid under pressure is mounted in the outside wall 14 of the monitor. In this figure, there can also be seen elements 18 for connection with the drill string and elements 20 and 22 for connection with the pressurized liquid pipe and with an annular air pipe that runs along the drill string so as to make it possible simultaneously to feed the nozzle with liquid and to feed an annular nozzle with air. According to that patent, the nozzle 16 is fed from the pipe 22 for feeding liquid under pressure via a passage 24 made in the body of the monitor 10 and via a tube 26 connecting the end of said passage to the inlet of the nozzle 16. The tube 26 is of constant section and of regular curvature so as to limit the disturbance it imparts to the liquid under pressure between the drill string and the nozzle 16 itself. Nevertheless, as explained, the diameter of the injection nozzle is very small compared with the diameter of the pipe that feeds liquid under pressure along the drill string. The solution described in the above-mentioned American patent is therefore not entirely satisfactory.
An object of the present invention is to provide an injection head, in particular for performing jet grouting, which makes it possible to further improve the quality of the jet delivered by the nozzle(s) of the injection head mounted at the bottom end of the drill string.
According to the invention, this object is achieved by a head for injecting liquid under pressure from a borehole to break up soil, said head being mounted at the end of a drill string, said string including liquid feed means for feeding liquid under pressure, said head comprising:
a body having an outside wall and a top end for connection to the bottom end of the drill string;
at least one injection nozzle mounted in said body, the inlet diameter of said nozzle being equal to d, said nozzle possessing an axis x,xxe2x80x2; and
duct-forming means for connecting the feed of liquid to the inlet of said nozzle, said duct-forming means presenting a mean line having a first end connected to the bottom end of said liquid feed means and having a second end connected to the nozzle tangentially to said axis x,xxe2x80x2, said mean line being defined by at least one curved portion presenting a radius of curvature that varies continuously, the right section of said duct-forming means decreasing regularly over at least half of its length from its first end to its second end.
It will be understood that the quality and the direction of the jet produced by the nozzle are substantially improved because the right section of the duct-forming means, e.g. a tube, decreases progressively and regularly, over at least half of its length from its end for connection to the pipe for feeding liquid under pressure to its connection to the inlet orifice of the injection nozzle. This characteristic combined with the fact that the mean line of the duct-forming means presents a radius of curvature which varies regularly makes it possible to minimize disturbances to the flow of liquid in said tube and thus to obtain a jet at maximum energy with erosive power that is maintained to a maximum distance away from the nozzle into the soil.
In a first embodiment, the axis of the liquid feed means coincides substantially with the axis of said body of the injection head, and the mean line of the duct-forming means comprises a first curved portion extending between said first end and an intermediate point and presenting regular curvature whose concavity presents a first sign, and a second curved portion extending between said intermediate point and said second end, and presenting regular curvature whose concavity presents an opposite sign.
It will be understood that because of the particular configuration of the duct-forming means, e.g. the tube, with its two curved portions, it is possible for given outside diameter of the injection head to increase the length of the tube and to increase the length of the tube portion that has a large radius of curvature connected directly to the nozzle.
In a second embodiment, the pipe for feeding under pressure extends into said injection head and the first end of the duct-forming means is a lateral branch on said pipe that is substantially tangential to said pipe.